1. Heat the oven to 325°F. Grease an 8- or 9-inch ceramic or glass baking dish or a deep dish pie plate with unmelted butter.

2. Put the butter, buttermilk, 1/2cup of the sugar, the egg yolks, flour, and salt in a blender or food processor and purée until smooth. Pour the batter into a bowl. Stir in the blueberries and zest and set aside.

3. In a separate bowl, beat the egg whites until they hold soft peaks, sprinkle in the remaining 1/4 cup sugar while beating until the whites hold stiff peaks; fold them into the batter gently but thoroughly.

4. Turn the batter into the prepared dish and put the dish in a baking pan large enough to hold it comfortably. Add enough warm water to the baking pan to come to within an inch or so of the top of the dish. Transfer carefully to the oven and bake until the top is golden brown and the center is just set but slightly jiggly, about 50 minutes.

5. Remove the cake from the oven and cool the dish completely on a rack, cover with plastic wrap, then refrigerate until chilled, at least 3 hours, before serving. This will keep in the refrigerator for 2 or 3 days.